In industries the use of metal products manufacturing by compaction and sintering metal powder compositions is becoming increasingly widespread. A number of different products of varying shape and thickness are being produced and the quality requirements are continuously raised at the same time as it is desired to reduce the cost. As net shape components, or near net shape components requiring a minimum of machining in order to reach finished shape, are obtained by press and sintering of iron powder compositions in combination with a high degree of material utilisation, this technique has a great advantage over conventional techniques for forming metal parts such as moulding or machining from bar stock or forgings.
One problem connected to the press and sintering method is however that the sintered component contains a certain amount of pores decreasing the strength of the component. Basically there are two ways to overcome the negative effect on mechanical properties caused by the component porosity. 1) The strength of the sintered component may by increased by introducing alloying elements such as carbon, copper, nickel molybdenum etc. 2) The porosity of the sintered component may be reduced by increasing the compressibility of the powder composition, and/or increasing the compaction pressure for a higher green density, or increasing the shrinkage of the component during sintering. In practise a combination of strengthening the component by addition of alloying elements and minimising the porosity are applied.
Powder forging includes rapid densification of a sintered preform using a forging strike. The result is a fully dense net shape, or near net shape, part suitable for high performance applications. Typically, powder forged articles have been manufactured from iron powder mixed with copper and graphite. Other types of materials suggested include iron powder prealloyed with nickel and molybdenum and small amounts of manganese to enhance iron hardenability without developing stable oxides. Machinability enhancing agents such as MnS are also commonly added.
Carbon in the finished component will increase the strength and hardness. Copper melts before the sintering temperature is reached thus increasing the diffusion rate and promotes the formation of sintering necks. Addition of copper will improve the strength, hardness and hardenability.
Connecting rods for internal combustion engines have successfully been produced by the powder forging technique. When producing connecting rods using powder forging, the big end of the compacted and sintered component is usually subjected to a fracture split operation. Holes and threads for the big end bolts are machined. An essential property for a connecting rod in a internal combustion engine is high compressive yield strength as such connecting rod is subjected to compressive loadings three times as high as the tensile loadings. Another essential material property is an appropriate machinability as holes and threads have to be machined in order to connect the split big ends after mounting. However, connecting rod manufacture is a high volume and price sensitive application with strict performance, design and durability requirements. Therefore materials or processes that provide lower costs are highly desirable.
U.S. Pat. No. 3,901,661, U.S. Pat. No. 4,069,044, U.S. Pat. No. 4,266,974, U.S. Pat. No. 5,605,559, U.S. Pat. No. 6,348,080 and WO03/106079 describes molybdenum containing powders. When powder prealloyed with molybdenum is used to produce pressed and sintered parts, bainite is easily formed in the sintered part. In particular, when using powders having low contents of molybdenum the formed bainite is coarse impairing machinability, which can be in particular problematic for connecting rods where good machinability is desirable. Molybdenum is also very expensive as alloying element.
However, in U.S. Pat. No. 5,605,559 a microstructure of fine pearlite has been obtained with a Mo-alloyed powder by keeping Mn very low. It is stated that, Mo improves the strength of steel by solution hardening and precipitation hardening of Mo carbide, and the like. However, when Mo content is less than about 0.1 wt %, its effect is small. Mn improves the strength of a heat-treated material by improving its hardenability. However, when Mn content exceeds about 0.08 wt %, oxide is produced on the surface of alloy steel powders such that compressibility is lowered and hardenability is increased beyond the required level. Hence, a coarse upper bainite structure is formed and strength is lowered. Keeping the Mn content low can however be expensive, in particular when using cheap steel scrap in the production, since steel scrap often contains Mn of 0.1 wt % and above. Thus a powder produced accordingly will be comparably expensive, due to low Mn content and the cost for Mo.
US 2003/0033904, US 2003/0196511 and US2006/086204, describe powders useful for the production of powder forged connecting rods. The powders contain prealloyed iron-based, manganese and sulfur containing powders, mixed with copper powder and graphite. US 2006/086204 describes a connecting rod made from a mixture of iron powder, graphite, manganese sulfide and copper powder. The highest value of compressive yield strength, 775 MPa, was obtained for a material having 3 wt % Cu and 0.7 wt % of graphite. The corresponding value for hardness was 34.7 HRC, which corresponds to about 340 HV1. A reduction of the copper and carbon contents also will lead to reduced compressive yield strength and hardness